As a member of the resident human microflora, the Gram-positive anaerobic coryneform bacterium Propionibacterium acnes (P. acnes) is found predominantly in the sebaceous gland of the skin. It can, however, also be isolated from the conjunctiva, the external ear canal, the mouth, the upper respiratory tract and, in some individuals, the intestine. P. acnes has an estimated skin density of 102 to 105-6 cm−2. P. acnes is a well-recognized opportunistic pathogen, especially in relation to medical implants such as central nervous system shunts, silicone implants and prosthetic hip joints. It is also responsible for ocular and periocular infections and endophthalmitis and has been implicated in periodontal and dental infections. Dental probing and treatment has lead to the dissemination of P. acnes in the bloodstream, which is a recognized cause of endocarditis in relation to damaged or prosthetic heart valves. P. acnes also plays a role in inflammatory acne, since antimicrobial therapy directed against P. acnes results in improvement, while the development of antibiotic resistance in P. acnes is associated with relapse. The common form of acne, known as acne vulgaris, affects up to 80% of the population at some time in their lives, making it the most common skin infection. There is also a strong association between severe forms of acne and joint pain, inflammation of the bone (osteitis) and arthritis. In patients suffering from this condition, known as SAPHO (synovitis, acne, pustulosis, hyperostosis and osteitis) syndrome, isolates of P. acnes have been recovered from bone biopsy samples, as well as synovial fluid and tissue.
Two distinct phenotypes of P. acnes, types I and II, have been identified based on serological agglutination tests and cell-wall sugar analysis. Recently, recA-based sequence analysis has revealed that P. acnes types I and II represent phylogenetically distinct groups (McDowell et al., 2005).
P. acnes produces a co-haemolytic reaction with both sheep and human erythrocytes (Choudhury, 1978) similar to the Christie-Atkins-Munch-Petersen (CAMP) reaction first demonstrated in 1944 (Christie et al., 1944). The CAMP reaction describes the synergistic haemolysis of sheep erythrocytes by the CAMP factor from Streptococcus agalactiae and the toxin (sphingomyelinase C) from Staphylococcus aureus, with the CAMP factor demonstrating non-enzymic affinity for ceramide (Bernheimer et al., 1979). Examination of sphingomyelinase-treated sheep erythrocytes has revealed the formation of discrete membrane pores by recombinant Streptococcus agalactiae CAMP factor (Lang & Palmer, 2003). In addition to the extensive study of the CAMP factor of Streptococcus agalactiae (Bernheimer et al., 1979; Brown et al., 1974; Jurgens et al., 1985, 1987; Ruhlmann et al., 1988; Skalka et al., 1980), a number of other Gram-positive and Gram-negative bacteria are known to produce a positive CAMP reaction, including Pasteurella haemolytica (Fraser, 1962), Aeromonas species (Figura & Guglielmetti, 1987), some Vibrio species (Kohler, 1988) and group G streptococci (Soedermanto & Lammler, 1996). Some of these species can also use phospholipase C (α-toxin) from Clostridium perfringens or phospholipase D from Corynebacterium pseudotuberculosis as a co-factor for haemolysis in addition to the Staphylococcus aureus toxin (Frey et al., 1989). The CAMP factor genes of Actinobacillus pleuropneumoniae and Streptococcus uberis have been identified, cloned and expressed in Escherichia coli (Frey et al., 1989; Jiang et al., 1996).
The precise role of the CAMP molecule in bacterial virulence remains unclear. It is likely that the co-haemolytic reaction represents a laboratory phenotype, or epiphenomenon, that is convenient for CAMP factor detection, but which may not be directly related to the role of the molecule in colonization and pathogenesis. The CAMP factor from Streptococcus agalactiae binds to the Fc region of IgG and IgM molecules, similar to the binding of IgG by Staphylococcus aureus protein A (Jurgens et al., 1987), and partial amino acid sequence similarity between the CAMP factor protein of Streptococcus agalactiae and Staphylococcus aureus protein A has been demonstrated (Ruhlmann et al., 1988).